The present invention is directed to DC motors, and more particularly to control of a DC motor used to spin a magnetic storage medium in relation to a read/write head assembly.
A magnetic storage medium is attached to a spindle that is rotated by a DC motor in a typical hard disk drive. Such DC motors are often conventional polyphase, brushless DC motors that are highly efficient and possess characteristics that make them suitable for a number of applications. Polyphase, brushless DC motors require application of voltage to armature windings in sequential order (i.e., commutation states) to produce torque-inducing flux for moving a rotor. For proper operation, the commutation state sequence must be timed to match the current rotor position at any point in time. Thus, for example, where a three phase, brushless DC motor is utilized, voltage is applied across pairs of armatures in a sequential order. The magnetic flux created by applying the aforementioned voltages across the armatures results in a torque on the motor that causes rotational movement. When the brushless DC motor is spinning, the timing for applying the aforementioned currents can be obtained by detecting zero crossings of the back electro motive force of the motor and locking to the zero crossings with an analog or digital locking circuit (e.g., a PLL or a DLL).
There are not any zero crossings to lock to when a DC motor is initially starting. Thus, another mechanism must be applied to govern the sequential timing of the applied voltages. In one particular case, the DC motor is initially forced to a known position. This can be done, for example, by applying a voltage to one or more randomly selected armatures to cause the motor to snap into a known position. This approach, however, has two potential drawbacks. First, when a current is applied to a randomly selected armature, the motor may move in a direction opposite that of the normal rotational direction. This is unacceptable for a disk drive that brings the read/write head assembly to rest on the spindle platter as a direction reversal can damage the read/write head assembly.
Second, to ensure that the DC motor moves to the proper position, the motor typically must be driven to two known states. Two states are required to cover the case where the initial rotor position is in a zero torque state for the state that is being driven. After each movement, a spindle associated with the motor will mechanically ring. To avoid the deleterious effects of such ringing, a delay is introduced between the two movements and after the second movement to allow the ringing to stop or at least diminish to an acceptable level. These delays can be substantial, and result in an undesirable delay in the time required to access a magnetic storage medium associated with the motor.
Hence, for at least the aforementioned reasons, there exists a need in the art for advanced systems and methods for controlling DC motors.